I agree that with the available data, it is impossible to know how many people use Bitcoin. However, there are other data that are no less important that can be analyzed, and these data are on-chain.

0xbitcoiner

bitcoin

Good metrics for Bitcoin adoption?

phaedrus

No amount of on-chain analysis can tell you how many unique humans are using it. Not even close. 1 exchange could be generating a new address for every transaction from every user, while another is sharing only one outgoing address for all of it's users! 

It's amazing to me that chainalysis ever gets anything right. I bet they're wrong more often. 